It is known in the art that the catalytic vapor phase fluorination of haloalkanes with hydrogen fluoride results in the formation of fluorine rich haloalkanes. Aluminum fluoride is one of the catalysts known in the art for the halogen exchange. However a suitable catalyst is required for the fluorination of haloalkenes to give fluorine rich haloalkanes.
As U.S. Pat. No. 2,885,427 (1959) has found CrF3 3H2O as a suitable catalyst for the fluorination of haloalkanes and haloalkenes, CrF3.3H2O is only precatalyst which is oxygenated at 600° C. to obtain an active catalyst whose empirical formula was found to be CrOF3F2. The reaction of trichloroethylene (herein after referred to as TCE), with HF in vapor phase at 350° C. using the above catalyst gave 2-chloro-1,1,1-trifluoroethane (herein after designated as HCFC-133a) as the major component and HFC-134a as a minor component.
The formation of HFC-134a from TCE involves several steps. The first step is the addition of HF as per Markownikov's rule to give 1-fluoro-1,1,2-trichlorethane (HCFC-131a). Subsequently, the chlorine present in HCFC-131a will be successively replaced by fluorine via the intermediates 1,2-dichloro-1,1, difluoroethane (HCFC-132b), HCFC-133a to give finally HFC-134a. It is known in the art that the ease of replacement of chlorine bound to a carbon, by fluorine follows the order trihalide (—CX3)>dihalide (—CHX2)>primary halide (—CH2X) where X=Cl. In the specific case of the catalytical fluorination of TCE a very high yield of HCFC-133a is obtained. However, the replacement of the primary halide present in HCFC-133a requires an efficient catalyst and relatively higher temperature to get good conversions and high selectivity, which are important for commercial preparation. It thus became necessary to divide the fluorination of TCE into two stages. The first stage involves fluorination of TCE to give HCFC-133a. The second stage involves the fluorination of HCFC-133a to give HFC-134a. UK Patent GB 2,030,981 A (1979) reported the fluorination of HCFC-133a at 400° C. using CrF3.3H2O as a precatalyst. The catalyst was activated by treatment first with air and then with a mixture of HF and air. After activation and during the initial period of fluorination, HCFC-133a and HF in a mole ratio of 1:6 were passed over the catalyst to obtain 31% conversion and 98% selectivity for HFC-134a. Subsequently, the reaction was continued by introducing additionally air during which time both conversion and selectivity started falling gradually.
The discovery of oxygenated CrF3.3H2O as a precatalyst lead to the development of several new catalysts based on the oxides of Chromium, Nickel, Cobalt, Aluminum etc. The patents U.S. Pat. No. 3,752,850 (1973), U.S. Pat. No. 3,859,424 (1975), described the use of Cr(OH)3 or Cr2O3.XH2O as a precatalyst which is activated by a process of calcination followed by fluorination with HF. The fluorination of TCE to give HCFC-133a was carried out at atmospheric pressure using a HF:TCE mole ratio of 6:1. The best conversion and selectivity were obtained at temperatures in the range 300° C.-340° C. The yield of HCFC-133a was 93%. The patents U.S. Pat. No. 3,755,477 (1973), U.S. Pat. No. 4,129,603 (1978) and U.S. Pat. No. 4,158,675 (1979) report a fluorination catalyst prepared by the sequence of precipitation of Cr(OH)3 from Cr3+ salts using a base, steam treatment at 95° C., dehydration, calcination and HF treatment. The U.S. Pat. No. 3,755,477 reports a yield of 85% HCFC-133a using HF:TCE in mole ratio 6:1 at 300° and atmospheric pressure. The U.S. Pat. Nos. 4,129,603 and 4,158,675 claim a highest conversion of 18.2% in the fluorination using HF:HCFC-133a in mole ratio 3:1, at a reaction temperature in the range 335°-355° C. and atmospheric pressure. The selectivity for HFC-134a was 91%.
There have been further modifications in the preparation of the precatalyst based on chromium hydroxide. The European Patent 0514932 (1992) described the preparation of Cr(OH)3 from Cr(NO3)3 with different surface areas in the range 48-180 m2/g and used graphite as an additive. This catalyst gave a maximum conversion of 20.3% with a selectivity of 95.7% for HFC-134a using HF:HCFC-133a in mole ratio 4.6:1, at a reaction temperature of 330° C. and a space velocity of 2250/h.
The EP 0546883 (1992) reported the preparation of chromia with or without Ni compound using sol gel technique. The addition of Nickel compound has improved the life of the catalyst.
The patents EP0486333 A1 (1991) and EP 0554165 A1 (1993) reported a catalyst containing chromia/Nickel salt impregnated on partially fluorinated Alumina or AlF3. The fluorination of HCFC-133a was carried out under pressure and in the presence of oxygen, to give HFC-134a with a maximum conversion of 21% and 99% selectivity.
The EP 0641598 A2 (1994) discloses a process for the fluorination catalyst by firing Cr(III) hydroxides in hydrogen atmosphere. The catalyst obtained was crystalline Cr2O3. The catalyst prepared in this Patent contains two stages using a mole ratio of HF:TCE 15:1 a conversion of 91.2% TCE and 95.3% selectivity for HCFC-133a was obtained. In the second stage using a mole ratio of HF:HCFC133a (8:1) a conversion of 19.8% HCFC-133a and 99.3% selectivity for HFC-134a was obtained. The catalyst obtained by the method of this invention has only two elements. The catalyst is crystalline and co-precipitation occurs at lower dilutions. On the other hand, the catalyst of the present invention contains three elements (Cr/Al/Zn) in which ZnCl2 is impregnated on a co-precipitated Chromia/Alumina catalyst, the catalyst is amorphous and co-precipitation has been done at higher dilutions.
The U.S. Pat. No. 4,792,643 (1988) Patent discloses a methodology for the preparation of HFC-134a starting from HCFC-133a using a catalyst prepared by co-extrusion of Aluminum oxyhydroxide and chromium oxide. The preparation of HCFC-133a from TCE using a catalyst prepared by co-extruded catalyst impregnated with cobalt chloride. The Patent reports the preparation of different catalyst by impregnation of CrO3, TiCl4, CrCl3, CoCl2 and NiCl2 on porous activated alumina. These catalysts were used to obtain directly HFC-134a by fluorination of TCE. The conversions of TCE and the combined selectivities for HFC-134a and HCFC-133a are low for large-scale preparations. In short, this patent described a methodology for the preparation of co-deposition of chromia and a compound of transition metal (Ti, Zr, Mo, Mg, Co, Ni) on alumina simultaneously or sequentially. This invention is also different from the present invention.
The U.S. Pat. No. 5,155,082 (1992) described a methodology for the preparation of co-deposition of chromia and a compound of transition metal (Ti, Zr, Mo, Mg, Co, Ni) on alumina simultaneously or sequentially. The Patent discloses a catalyst prepared by blending Al(OH)3 and chromium oxide in the presence of a solvent. This catalyst after calcination and fluorination was used in the reactions of HF, separately with TCE and HCFC-133a under pressure. In the case of TCE, high selectivity for HCFC-133a was reported although no values were given. The fluorination of HCFC-133a was reported to give 18% conversion with 94% selectivity for HFC-134a. In short, the above Patent discloses a methodology for the preparation of HFC-134a starting from HCFC-133a using a catalyst prepared by co-extrusion of Aluminum oxyhydroxide and chromium oxide. The preparation of HCFC-133a from TCE using a catalyst prepared by co-extrusion of aluminum oxyhydroxide/chromium oxide or the co-extruded catalyst impregnated with cobalt chloride. This invention is entirely different from the present process especially in co-extrusion or co-deposition catalyst.
The EP 0328127 A1 (1989) reports the use of a catalyst obtained by impregnation of compounds of Co, Mn, Ni, Pd, Ag and Ru on alumina or AlOF as a precatalyst for the fluorination of HCFC-133a. The catalyst obtained from CoCl2/Al2O3 gave conversion of 33.5% with selectivity 93.7% for HFC-134a in the fluorination of HCFC-133a using HF containing ppm levels of oxygen. The above catalyst has been further modified in Indian Patent 172054 (1989) by using additives selected from compounds of metals having atomic number 58-71. At temperature above 350° C. and using HF:HCFC-133a mole ratio in the range 10:1 to 20:1, conversions in the range 30-40% were obtained. At higher temperatures the conversions were higher but the selectivity dropped to 82.9%.
The patents WO 92/16480 (1992) and WO 92/16481 (1992) disclosed a new catalyst prepared by impregnation of zinc compound on Al2O3 and optionally containing one or more other metal selected from this group with atomic number 57-71. This catalyst was used for fluorination of TCE and also HCFC-133a to obtain very high selectivities for HCFC-133a and HFC-134a respectively. However, very high contact times are required in the fluorination of TCE.
Another publication to Rao J. M. et al teaches the effect of acid strength of co-precipitated chromia/alumina catalyst on the conversion and selectivity in the fluorination of 2-chloro 1,1,1-trifluoroethane to 1,1,1,2-tetrafluoroethane. (Journal of Fluorine Chemistry 95, pp. 177-180 Elsevier Science 1999). The present invention is entirely different and not related to subject matter disclosed by this document. An examination of TPR data reveals a difference in reduction pattern in terms of T-max variation and also hydrogen uptake per mmol.g−1. This data further supports the increased number in the availability of reducible group in the catalyst B of the present invention in comparison to catalyst B of Rao et al, thereby providing enhanced selectivity and efficacy in the formation of the required product 1,1,1,2-tetrafluoroethane in the present invention.
The use of compounds of zinc and/or magnesium as promoters on chromium based catalyst impregnated on Alumina or AlF3 was reported in the EP 0502605 A1 (1992). In fluorination using HF:TCE in a mole ratio of 10:1, a conversion of only 40.9% was reported at 310° C. and contact time of 1 sec. The same catalyst gave a conversion of 20.5% with a selectivity>99% in the fluorination using HF:HCFC-133a in mole ratio 3.5:1 at reaction temperature of 330° C. and contact time 2 sec.